Fix flaky test based on Nondex

[SpiceOwlf]

Hi, I used Nondex(https://github.com/TestingResearchIllinois/NonDex) on the project and found [checkTemplateNegociationDefault] does not have a uniform behavior. So I made some changes to the test to make it pass all possible situations. Please let me know if you want to discuss this change more. Thanks.

[SpiceOwlf]

Updates:
The root cause of the failure is that (1) the templates are read from thedisk using listFiles() in

redpipe/redpipe-engine/src/main/java/net/redpipe/engine/template/AbstractTemplate.java

Line 88 in 0aff891

return fs.rxReadDir(templateDir)

and (2) the files are put in a HashMap of variants in

redpipe/redpipe-engine/src/main/java/net/redpipe/engine/template/AbstractTemplate.java

Line 94 in 0aff891

Map<Variant, String> variants = new HashMap<>();

Neither listFiles() nor HashMap guarantee the order of the
elements, so the order of the variants in nondeterministic. When the
default variant is chosen as the first from the map it can end up being
one of the two variants, while the assertion expects it to be only one.